With the decline of fossil fuel reserves and the sudden increase and change of oil prices, research is being conducted for an improvement in the fuel efficiency of vehicles. Important for fuel efficiency improvement are the weight reduction design of vehicle bodies and the minimization of power loss by reducing frictions at system links. Additionally, the maximization of output efficiency by improving dynamic characteristics upon the exhaustion control of the engine itself contributes to fuel efficiency. In regard to the improvement of fuel efficiency, research has been conducted to reduce a dynamic load through the weight reduction of dynamic components of the engine head.
Of the dynamic components, an engine valve spring of a vehicle is a component that contributes to fuel efficiency when the weight thereof is reduced, because it directly controls a dynamic load. Conventionally, valve springs have been made mainly of chromium silicide (CrSi) steel that has a tensile strength of 1900 MPa or chromium silicide vanadium (CrSiV) steel that has a tensile strength of 2100 MPa. Recently, attempts have been made to increase the tensile strength of the steel for the engine valve spring to a level of 2550 MPa by adding alloy elements to CrSiV steels.